1. Field of the Invention
The present invention relates to a photoelectric conversion device, a method of manufacturing the same, and an image sensing system.
2. Description of the Related Art
Recently, digitization of cameras is accelerating, and digital cameras including a photoelectric conversion device for photoelectrically converting an object image formed by an image sensing optical system are now widespread. Typical examples of the photoelectric conversion device are a CCD image sensor and a CMOS image sensor such as APS (Active Pixel Sensor). The CMOS image sensor is expected to have wide applicability because it can select a pixel using an address system and readily achieves system-on-chip.
The CMOS image sensor generally uses a photodiode (buried photodiode) having a carrier accumulation region buried in the substrate to suppress a dark current generated in the surface. In the buried photodiode, however, the carrier accumulation region is arranged in the substrate far apart from its surface. This raises the threshold voltage of a transfer transistor.
Japanese Patent Laid-Open No. 11-274454 discloses a technique of providing, between the carrier accumulation region of a photodiode and the control electrode of a transfer transistor, a region (to be referred to as a “bypass region” hereinafter) of the same conductivity type as the carrier accumulation region and increasing the impurity concentration in a floating diffusion region. According to Japanese Patent Laid-Open No. 11-274454, the depletion layer extends from the floating diffusion region to the bypass region. In this state, the transfer transistor is designed to transfer electric carriers from the carrier accumulation region to the floating diffusion region via the bypass region having a low potential. This supposedly makes it possible to lower the threshold voltage of the transfer transistor.
In the technique disclosed in Japanese Patent Laid-Open No. 2003-258231, the control electrode of a transfer transistor is formed from a main body portion and an expansion portion that is arranged adjacent to a photodiode with respect to the central portion of the main body portion. According to Japanese Patent Laid-Open No. 2003-258231, the expansion portion partially increases, to the photodiode side, the gate length of the central portion of the main body portion of the control electrode of the transfer transistor. This probably allows to read out signal from the carrier accumulation region of the photodiode by complete transfer.
However, the present inventor found the following facts. In the invention of Japanese Patent Laid-Open No. 11-274454, the bypass region is provided between the carrier accumulation region of the photodiode and the region under the control electrode of the transfer transistor. Hence, the distance from the control electrode of the transfer transistor to an end portion of the carrier accumulation region of the photodiode tends to increase. Since the depletion layer hardly extends all over the carrier accumulation region, signal charges may remain at part of the carrier accumulation region.
In the invention of Japanese Patent Laid-Open No. 2003-258231, the expansion portion partially increases the gate length of the central portion of the main body portion of the control electrode of the transfer transistor toward the center of the photodiode. This makes it difficult to photoelectrically convert light that has entered near the center of the photodiode. Since the light amount is large near the center of the photodiode, the sensitivity of the photodiode degrades.